1. Field of the Invention
The present invention relates to a direct cylinder injection-type spark-ignition internal combustion engine.
2. Prior Art
There has heretofore been known stratified charge combustion produced by directly injecting fuel into a cylinder to form a mixture only in the vicinity of a spark plug at the ignition timing to burn a lean mixture in the cylinder as a whole. To carry out the stratified charge combustion, in general, the fuel injection valve is opened only for the period required for injecting a required amount of fuel from a crank angle, for starting the fuel injection, set in the latter half of the compression stroke. It is intended that the thus injected fuel proceeds into a concave combustion chamber formed in the top surface of the piston, is deflected toward the spark plug due to the shape of the combustion chamber while being vaporized by robbing heat from the wall surface of the combustion chamber and forms a mixture that can be favorably ignited near the spark plug.
In general, the fuel injection valve injects fuel in a form that is conically concentrated. Therefore, a relative long period is required while the fuel injected in the last stage of fuel injection becomes a combustible mixture utilizing the heat from the wall surface of the combustion chamber. To ensure this period, therefore, the fuel injection end timing must be advanced. An amount of fuel which can be injected in the latter half of the compression stroke, therefore, inevitably decreases, and stratified charge combustion must be abandoned in high engine load operations in which a relatively large amount of fuel is required. It has therefore been desired to carry out stratified charge combustion, which is effective in decreasing the consumption of fuel, over a wider range of engine operations.
Japanese Unexamined Patent Publication (Kokai) No. 9-158736 proposes injecting the fuel in the shape of a flat fan having a relatively small thickness by using a fuel injection valve having an injection hole in the shape of a slit. The thus injected fuel can rob heat from a wide areas of the wall surface of the combustion chamber, making it possible to form a combustible mixture within a short period and to retard the timing for ending the injection of fuel. It is therefore possible to increase an amount of fuel injected in the latter half of the compression stroke and to expand the region of stratified charge combustion toward the high engine load side.
According to the prior art as described above, even if the timing for ending the injection of fuel is retarded, a combustible mixture can be reliably formed from the injected fuel at an ignition timing. The thus formed combustible mixture has a flat shape with its length being shorter than the width thereof and rises nearly in the direction of the length. The ignition timing must occur while the combustible mixture is rising and is coming into contact with the spark plug. However, the combustible mixture has a relative short length and stays contacted to the spark plug while it is rising for only a relative short period. Therefore, it may often happen that the combustible mixture has already passed over the spark plug at the ignition timing due to a slight deviation in the timing for forming the combustible mixture, and reliable ignition performance is not maintained.